Kimberly Weirich

Bilayer edges catalyze supported lipid bilayer formation.

Supported lipid bilayers (SLB) are important for the study of membrane-based phenomena and as coatings for biosensors. Nevertheless, there is a fundamental lack of understanding of the process by which they form from vesicles in solution. We report insights into the mechanism of SLB formation by vesicle adsorption using temperature-controlled time-resolved fluorescence microscopy at low vesicle concentrations. First, lipid accumulates on the surface at a constant rate up to approximately 0.8 of SLB coverage. Then, as patches of SLB nucleate and spread, the rate of accumulation increases. At a coverage of approximately 1.5 x SLB, excess vesicles desorb as SLB patches rapidly coalesce into a continuous SLB. Variable surface fluorescence immediately before SLB patch formation argues against the existence of a critical vesicle density necessary for rupture. The accelerating rate of accumulation and the widespread, abrupt loss of vesicles coincide with the emergence and disappearance of patch edges. We conclude that SLB edges enhance vesicle adhesion to the surface and induce vesicle rupture, thus playing a key role in the formation of continuous SLB.

Simulation of edge facilitated adsorption and critical concentration induced rupture of vesicles at a surface

We investigate the kinetics of supported lipid bilayer formation by the adsorption and rupture of uncharged phosphatidylcholine lipid vesicles on to a solid substrate. We model the adsorption process taking into account the distinct vesicle rupture events and growth processes. This includes (i) the initial adhesion and vesicle rupture that nucleates bilayer islands, (ii) the growth and merger of bilayer islands, (iii) enhanced adhesion of vesicles to the bilayer edge, and (iv) the final desorption of excess vesicles from the substrate. These simulation studies give insight into prior experimental observations of adsorption in which an overloading of lipid on the solid substrate occurs before formation of the final supported lipid bilayer. Our model provides an explanation for the features of the interesting universal master curve that was observed for the surface fluorescence intensity in the experimental investigations of Weirich et al.

Thermal Stress of Supported Lipid Bilayer Induces Formation and Collapse of Uniform Radius Tubules

Lipid bilayer morphologies and the transitions between them are important to many cellular processes. Supported lipid bilayer (SLB) provides a model system in which to quantitatively investigate transitions from planar to tubular and tubular to spherical morphologies. Following a small increase in temperature (∼5-10°C) flexible filaments extrude from a fluid SLB. Individual filaments can reach hundreds of microns in length before spontaneously collapsing into discs. We demonstrate that the filaments are tubular by decreasing the external buffer concentration, which causes them to swell, first into resolvable tubules with capped ends and then into giant vesicles. At high ionic strength, the sub-resolution tubules are adsorbed to the SLB, enabling the measurement of their radius to within ±5 nm using conventional fluorescence microscopy. The radius depends on the lipid tail composition and varies <10% along the tubule length. Under tension, tubules are even more uniform, having no measurable variation in radius.